Note: Roof available on Car-part.com for $450!!!The S6 is the performance variant of Audi's midsize sedan, the A6, and competes against the BMW M5 and Mercedes-Benz E63 AMG. While there was once an even-more audacious 6-based rocket called the RS 6, currently only the S version is sold in the U.S. With a V-10 crammed under its hood, performance-tuned all-wheel drive and a modified suspension, the S6 is a thrilling performer that hides its capabilities under a classy but unobtrusive wrapper. Audi lane assist is a new option for 2008, but little else has changed.